1. Field of the Invention
The present invention relates to a toner for use in an image-forming method which utilizes electrophotographic technology, electrostatic recording technology or toner jet recording technology. More specifically, the invention relates to a toner for use in an image-forming method in which a toner image is formed on an electrostatic latent image-bearing member, then is transferred onto a transfer material to form a toner image which is subsequently fixed under heat and pressure to obtain a fixed image.
2. Description of the Related Art
How to implement energy savings in the field of copiers and printers has become a major technical concern in recent years. One approach that has arisen is to dramatically reduce the amount of heat applied to the fixing apparatus in electrophotographic equipment. This has led in turn to an increased need in toners for “low-temperature fixability” which enables sufficient fixing of the toner to occur at a lower energy.
One method that is known to be effective for enabling fixing to occur at lower temperatures is to confer the binder resin with sharp melting properties that allow the resin to melt under a small temperature change. It is in this connection that toners which use crystalline polyester resins have been proposed. Crystalline polyesters, because they have the property—owing to the regular arrangement of the molecular chain—of not exhibiting a distinct glass transition and not readily softening up to the crystal melting point, are being investigated as a material which can be endowed with both heat-resistant storage stability and low-temperature fixability.
WO 2009/122687 discloses a toner obtained by a dissolution suspension method wherein a block polymer which uses a polyester resin, a polyurethane resin, a polyurea resin, a polyamide resin or a polyether resin in crystalline segments and non-crystalline segments is used as a binder resin.
This disclosure describes, for the block polymer, about control of the viscoelastic behavior at the endothermic peak temperature Ta from the block polymer in heat of fusion measurement using a differential scanning calorimetry (DSC) and in the temperature range around the melt onset temperature X in a Koka-type flow tester.
When a crystalline polyester is used in the binder resin, sharp melting properties can be imparted to the toner. However, owing to inadequate viscosity during melting of the toner, hot offset readily arises in the fixing step on the high temperature side.
In the case of toners having a core-shell structure, it is conceivable to introduce a crystalline structure into the shell material itself.
Japanese Patent Application Laid-open No. 2010-150535 introduces a large number of structures capable of forming crystallinity, such as long-chain alkyl groups and crystalline polyester units, into the shell material, thereby conferring the shell material with sharp melting properties, and attempts in this way to endow the toner with both low-temperature fixability and heat-resistant storage stability. However, it has been found that this approach makes it difficult to maintain the viscosity during melting of the toner, leading to an inadequate hot offset resistance.
As a result, in toners having a core-shell structure, it is necessary not only to confer sharp melting properties, but also to suppress a decline in the viscosity of the overall toner due to melting of the binder resin.
Japanese Patent Publication No. 4285289 discloses, in a toner, which is obtained by agglomeration method, containing crystalline structures in the core, the art of utilizing metallic ions within an agglomerating agent for inducing agglomeration of the fine particles in order to effect crosslinking between molecular chains of the resin, and thereby retaining the high-temperature side viscosity of the toner. In this way, the viscosity of the binder resin during melting of the toner is retained, enhancing the temperature region in which fixing is possible.
However, it has been found that, in this method, because the molecular chains are strongly bonded chemically by ionic crosslinking, the decrease in viscosity during melting of the toner is suppressed, making it difficult to enhance the fixing temperature region.
Hence, there exists a need to carry out technical improvements in such a way as to not only impart sharp melting properties to the shell material, but also suppress a decrease in the viscosity of the shell material during melting of the toner on the high-temperature side in the fixing step, and thus ensure a decrease in the viscoelasticity of the overall toner.